To slow the rising number of infections – tens of thousands of Americans have been infected since Jan. 20 – researchers are looking for the coronavirus' limits. Just how long can it last outside the human body?

The report also noted the half-life, or rate of decay, of the virus on various materials. That's the time it took for half of the virus sample to die.

The decay rate is important because though the virus may linger on surfaces for days, people are less likely to become infected as the virus dies.

Most infections start with water droplets, tiny globes of water 5 microns or less in size. That's much smaller than a human hair, which is 60 to 120 microns thick.

In all settings, viruses need water to survive. "Viruses can withstand a small amount of dehydration," says Dr. Paul Meechan, a former director of safety at the Centers for Disease Control and Prevention and president of the American Biological Safety Association.

The problem is knowing "how long it will take a virus to dry out and become noninfectious," Meechan says. "Eventually, the virus deteriorates and becomes inactive." The speed at which that happens depends on whether the virus is in the air or on a surface.

Let's start with air. Researchers don’t know how many virus-laden particles people infected with COVID-19 expel in the average droplet. An average cough, however, can produce as many as 3,000 droplets and a single sneeze can make up to 40,000, according to multiple studies cited in a 2009 World Health Organization report.

A droplet’s size determines where it goes after being expelled. Gravity forces larger ones to the ground.

"Aerosols are different," says Dr. Stanley Deresinski, clinical professor of medicine and infectious diseases at Stanford University. "Very small particles may be suspended in the air for a long time, sometimes for hours. They're suspended by air currents."

Airborne droplets can stay suspended long enough for someone to walk through and inhale the virus. Outdoors, wind disperses the virus.

A virus that doesn't reach the ground or floor can fall on shared surfaces – or be transferred there by those with the pathogen on their hands. Whatever the case, unsuspecting people can pick it up. How long a virus lives depends on the surface it's on:

How long coronavirus lives on steel

The coronavirus can exist on stainless steel objects for two to three days. That's a problem because steel is commonly used in public transportation and in scores of other public places such as restroom stalls, faucets, and manual paper towel handles.

How long coronavirus lives on plastic

Plastic objects can harbor the virus for two to three days. That's a special concern because many shared items are made of plastic and may not be sanitized often enough or completely enough. Take out food containers, light switches, cellphone cases, elevator buttons and more are commonly made of plastics.

How long coronavirus lives on cardboard

The virus can last on cardboard for up to 24 hours. That's noteworthy because many customers are using online delivery services during the coronavirus outbreak instead of going to stores in person. Food products packaged in cardboard could also be a risk.

How long coronavirus lives on glass

Corrections & clarifications: The above graphic previously misstated stated the time that COVID-19 could survive on glass.

A virus can last as long as four days on glass, depending on location and temperature, according to a separate study by the Journal of Hospital Infection published in January. That report charted the persistence of the SARS-CoV virus, which is similar to the current virus causing COVID-19. Items such as cellphone screens, mirrors and inside glass doors can also support the virus.

How long coronavirus lives on copper

The coronavirus lasted about four hours on copper, a finding consistent with historical use.

"Copper has been used for years," says Meechan. "Copper ions have been used as disinfectants, they're an effective virucide. It's one of the reasons why old doorknobs were made of brass."

Hospitals are increasing copper use in patient settings, including rails on hospital beds and other shared items.

This is how the reports calculate the survival rate of viruses, in hours, on non-disinfected surfaces:

Corrections & clarifications: The above graphic previously misstated stated the time that COVID-19 could survive on glass.

Cleaning these materials

The CDC defines cleaning as the removal of germs, dirt and impurities from surfaces. Disinfecting involves the use of chemicals to kill pathogens on surfaces. The agency says surfaces should be cleaned using disposable gloves, then disinfected to lower infection risk.

The Environmental Protection Agency has a list of disinfectant products that meet their criteria for use against SARS-CoV-2, the novel coronavirus that causes the disease COVID-19.

SOURCE USA TODAY reporting; New England Journal of Medicine; Stanford University; Centers for Disease Control and Prevention; World Health Organization; National Center for Biotechnology Information; Department of Energy, National Accelerator Laboratory